"A",
"B",
"C",
"D",
"E",
"F",
"G",
"H",
"I",
"J",
"K",
"L",
"M",
"N",
"O",
"P",
"Q",
"R",
"S",
"T",
"U",
"V",
"W",
"X",
"Y",
"Z"
Previous set of "D" definitions and terms.
DOPPLER EFFECT: In radar, the change in frequency of a received signal caused by the relative
motion between the radar and the target. The apparent change in frequency of an electromagnetic wave caused by a change in distance between the transmitter and the receiver during transmission/reception. The figure illustrates the Doppler increase that would be realized by comparing the signal received from a target approaching the radar site to the transmitted reference signal. An apparent frequency decrease would be noted for targets departing the radar location. Differences can be calibrated to provide target
range-rate data.
DOPPLER FREQUENCY: Difference between transmitted and reflected frequencies; caused by the
Doppler effect.
DOPPLER SHIFT: The degree of observed change in
frequency (or wavelength) of a wave due to the doppler effect.
DOUBLE-MODING: In a transmitter output tube, the abrupt and random change from one frequency to
another.
DRY-AIR SYSTEM: Provides dehumidified air for electronic equipment that is moisture critical.
DUCTING: Trapping of an rf wave between two layers of the earth's atmosphere or between an
atmospheric layer and the earth. The increase in range that an electromagnetic wave will travel due to a temperature inversion of the atmosphere. The temperature inversion forms a channel or waveguide (duct) for the waves to travel in, and they can be trapped, not attenuating as would be expected from the radar equation.
DUMMY LOAD (Radio Transmission): A dissipative but essentially non-radiating substitute device having
impedance characteristics simulating those of the antenna. This allows power to be applied to the radar transmitter without
radiating into free space. Dummy loads are commonly used during EMCON conditions or when troubleshooting a
transmitter at a workbench away from it's normal environment.
DUPLEXER: A radar device that switches the antenna from the transmitter to the receiver and vice
versa. A switching device used in radar to permit alternate use of the same antenna for both transmitting and receiving.
DUTY CYCLE: In a transmitter, ratio of time on to time off. The ratio of average power to peak power, or ratio of pulse length to interpulse period for pulsed transmitter systems.
Interpulse period is equal to the reciprocal of the pulse repetition rate.
